#include<stdio.h>
double cd(double i,double j)
{
	double l= (1/i + 1/j)/2;
	return 1/l;
}
int main()
{
	double average,i,j;
	printf("number:");
	scanf("%lf%lf",&i,&j);
	average = cd(i,j);
	printf("%lf",average);
}
